The origin of music is not a historical topic.

If you are after the oldest known notated piece of music, check this previous question for a good contender.

How would that stuff sound? That is a very difficult question to answer with certainty.

There have been many attempts to recreate the Hurrian songs and Seikilos epitaph, youtube will give you many examples of both.

There are many educated (and not so educated) guesses when attempting to reconstruct this stuff, we can't really think we have managed to hear "the real thing."

That music is very distant from us but, in the great scale of things, is not extremely old.

Im currently reading a book called "The Music Instinct" by Philip Ball where it details how music affects the human body/mind etc.

Early on he notes how looking through the various cultures and civilizations, mankind has developed music simultaneously without any contact with another civilization. While this may have evolved into many different types of music based on the culture, lifestyle, available goods around them i.e. the heavy bass of the African nomadic tribes using hollow logs and rods, the strings of ancient greek, the didgeridoo and clapping sticks of the Australian Aboriginees etc every has some kind of music.

In fact in his studies he has found no single culture to be devoid of music, even the deaf and blind will tap their feet to an invisible "beat" in their minds.

Why mankind has developed with music engraved into its brain is unknown, but it has been there since earliest recorded history. So asking what was the first song is kind of a moot point since it wasn't recorded, so far back has music been intertwined in our history.